Studies to elucidate the basis of H.C. Blue 1's carcinogenic activity have continued. H.C. Blue 1 is carcinogenic in the 36 mouse liver, but not in the F344 rat, whereas the structural analogue, H.C. Blue 2, is not carcinogenic in either species. During this year we have demonstrated that impurities in H.C. Blue 1 are responsible for its Salmonella mutagenicity, and DNA adduction. Thus, evidence is accumulating to indicate that a nongenotoxic mechanism may account for H.C. Blue 1's carcinogenic activities. Metabolic studies using [14C]-methy- and ring-labeled H.C. Blue 1, and [14C]-ring labeled H.C. Blue 2 have been conducted with B6 mice and hepatocytes from the B6 mouse. The studies have indicated that demethylation is a major pathway for the carcinogen, H.C. Blue 1, whereas oxidation to an acid is the major metabolic pathway (greater than 90%) for H.C. Blue 2. How this difference in metabolic pathways could contribute to differing carcinogenic activities, and whether purified H.C. Blue 1 can form DNA adducts in the liver or hepatocytes in vitro are being pursued.